Some modern communications solutions are based on VoIP (Voice-over IP (Internet Protocol)) technology, which involves the transmission of calls over a data network based on the IP. The communication is in the form of packet data and there is no fixed connection as there would be in the case of switched networks. The communication can be text, voice, graphics or video. In order to simplify IP communication problems, standards have been developed and adopted in the industry. Examples of such standards are H.323 (Packet based communication systems) and SIP (Session Initiation Protocol). These standards are followed when designing new hardware and software. The SIP standard covers the technical requirements to set-up, modify and tear down multimedia sessions over the Internet. A multimedia communication session between two endpoints will be referred to as a call.
In distributed networks, telephone terminal sets use both unicast and multicast messages to communicate with each other. For a network containing a number of peer terminal sets each providing local call processing functionality, the peer terminal sets communicate with each other using messaging sequences. However, there are no efficient mechanisms for communication between the terminal sets. Multicast messaging provides an efficient mechanism for distributing information. However, for a system having peer terminal sets on more than one network segment a difficulty arises in multicast messaging between the peer terminal sets. This is particularly problematic when two peer terminal sets are on different networks that are protected by firewalls. In such a case, the two terminal sets may have the same private IP address and there is no simple mechanism for communicating with each other. Furthermore, communication between the two peer terminal sets might require that messages be routed through a router; however, many routers do not support multicast messaging. Some systems have been developed to provide a mechanism for tunneling multicast messages between routers; however, such systems are very expensive and are not well suited for distributed networks.